Modelling study for assessment of strata behaviour in bord and pillar working under soft floor condition G. S. P. Singh* 1 and U. K. Singh 2 Safe and successful mining operation in bord and pillar workings under soft floor conditions requires a proper understanding of strata behaviour for better planning of mining layouts and selection of suitable mining horizon for better strata control and management of conditions arising due to excessive roof–floor convergence in the workings. This paper illustrates a modelling approach to assess the caving behaviour of strata, stress concentration, and roof–floor reactions in a depillaring bord and pillar working of Singareni coalfields in India. Assessment of floor heaving and roof–floor convergence in junctions of advance galleries is conducted with progressive goaf exposure. The calibrated model is used to predict the strata behaviour and roof– floor reactions in another depillaring panel. The modelling study has been helpful in better understanding of strata control issues for a confident decision making to ensure better safety and productivity of mine workings. Keywords: Bord and pillar, Soft floor, Strata behaviour, Main fall, Periodic caving, Floor heaving, Numerical modelling Introduction Bord and Pillar mining is one of the oldest and most popular methods of underground coal mining in India. Sustainable strata control is very important to ensure safety and optimal exploitation of coal seam from such workings. Many cases of significant mining problems have been reported from such workings, especially those located in soft and weak floor strata conditions. These range from local operational problems like bogging of equipment and support systems, floor heave leading to disruption in transportation and ventilation, regional deformation and collapse because of pillar settlement, pillar punching or indirect pillar or roof collapse through to long term unplanned surface subsidence. Modelling study of rock mechanics issues related with these problems can be helpful in proper design of mining layouts and selection of suitable sections for effective working of coal seams under such strata conditions. A number of attempts have been made by various researchers worldwide to understand the behaviour of soft floor and its implication on safety of bord and pillar workings. Damodaran 1 formulated an empirical model to study the geomechanical behaviour of bord and pillar workings under soft floor condition at Cooranbong Colliery, Australia and concluded that the thickness of coal left in the floor may increase bearing capacity of a soft floor. Khan 2 conducted a numerical simulation study to investigate the influence of soft floor strata on load redistribution and associated displacement in longwall mining for panel 701 at Southern Colliery, Australia; and analysed different soft floor thickness in terms of intensity and location of the peak front abutment pressure. The modelling study conducted by Dutton 3 shows that presence of moderately weak claystone in the roof of a pillar causes a reduction in confinement forces due to reduction of frictional and cohesion. This reduces the overall pillar strength which has a serious implication on safe design of pillars under such conditions. Godavari Khani 5A mine of Singareni Coal Company Ltd has worked a number of bord and pillar panels at a depth of ,90 m to extract 2?17 m coal along roof of a 6?37 m thick coal seam leaving the rest portion compris- ing mainly of clay, coal and shale in the floor. 4 The workings developed in this pattern are facing acute instability problem due to excessive heaving and degrada- tion of floor. During working of a number of panels such as panel 30A, caving in the goaf was followed by observance of daily convergence varying from 2 to 17 mm. The workings in these panels had to be tem- porarily discontinued during such periods to ensure strata control safety in the working area if the rate of con- vergence increases to 2 mm per day fearing the likely danger of instability of working area under this condition. Such periodic discontinuance of working during expected periods of major roof caving created a considerable loss of production and productivity of the workings in addi- tion to reduction in confidence of the workers towards general safety of the underground mine workings. Figure 1 shows the general layout of the panels including 1 Department of Mining Engineering, Institute of Technology, Banaras Hindu University, Varanasi 221005, India 2 Department of Mining Engineering, ISM University, Dhanbad 826004, India *Corresponding author, email gspsingh@yahoo.com ß 2009 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 25 June 2009; accepted 25 September 2009 DOI 10.1179/037178409X12541250836905 Mining Technology 2009 VOL 118 NO 2 91